1. Field of the Invention
The present invention belongs to the field of methods and processes for forming on the surface of metallic elements a protective layer which is corrosion resistant, which acts as a barrier layer preventing spontaneous interreaction between the material of the element and a coating disposed thereon, and which is endowed with specific physical characteristics or qualities different from those of the base material. Although products obtained by way of the present invention have a general usefulness as a result of being provided with a corrosion and electrical resistant surface film they are particularly useful as support members for photo-lithographic plates and the like.
The protective surface layer is obtained by an anodic electrolytic process.
2. Description of the Prior Art
Photo-lithographic plates currently in use today often include a metallic support member, having, for example, aluminum as its principal component, a surface of which has been silicated by chemical methods to provide a barrier layer which prevents interreaction between the photosensitive diazonium salts, or other photosensitive and non-photosensitive coatings, placed upon the support member and the metal surface of the support member. Silication of the metal surface provides a chemical pacification which increases the shelf life of the lithographic plate, facilitates the processing of the plate after exposure, and improves the length of the printing run and the quality of print. The barrier layer is obtained, according to the prior art, by subjecting the metallic surface to the action of a solution of one or several of a plurality of compounds, examples of which include hydrolized cellulose ester, sodium phosphate glass, alkali metal silicates, sodium metaborate, phosphomolybdate, sodium silicate, silicomolybdate, water-soluble alkylated methylomelamine formaldehyde, polyalkylene-polyaminemelamine-formaldehyde resins, urea-formaldehyde resin plus polyamide, polyacrylic acid, polymethacrylic acid, sodium salts of carboxymethylcellulose, carboxymethylhydroxyethyl-cellulose, zirconium hexafluoride, etc.
An often used solution in the prior art is an aqueous solution of sodium silicate in which the metallic plate, forming the lithographic plate support member, is dipped, or which is applied to a surface of the plate. The solution is preferably heated before dipping the plate therein or before applying to the surface of the plate, and the plate surface is optionally washed with an acidic medium in order to harden the silicated surface and neutralize any alkali that may remain on the surface.
In addition to acting as a barrier layer between the metal of the metallic plate and the diazo resin, the silicated surface forms a hydrophilic surface which partially acts as an initial water-carrying surface when the processed plate is placed in a printing press. The hydrophilic surface thus formed is desirably relatively insoluble in the fountain solutions used in a printing press in order to prevent undercutting or hydration of the image areas.
It has been postulated that the following reactions take place during conventional silication of an aluminum surface:
(1) The aluminum and the aluminum oxide at the surface of the plate react with the solution according to the formulae: EQU Al+20H.fwdarw.AlO.sub.2 +H.sub.2 (a) EQU Al.sub.2 O.sub.3 +20H.fwdarw.2AlO.sub.2 +H.sub.2 O (b) PA1 (2) Silication, simultaneously or consecutively, takes place at the surface, according to the following formula: EQU Al+AlO.sub.2 +SiO.sub.3 .fwdarw.(Al.sub.2 SiO.sub.5)2x
The aluminum silicate surface layer thus formed is substantially insoluble, although it may be dissolved to some extent in strong reagents, and it has been postulated that it is in the form of large super crystals having an endless chain-like structure as follows: ##STR1##
However in addition to aluminum silicate, other compounds may be formed and included in the surface layer, which often result in differences in the qualities of the surface layer. Some of the compounds that may be present in the film of aluminum silicate including Al (OH).sub.3, hydrated Al.sub.2 O.sub.3, and hydrated sodium aluminum silicate, such as, for example, Na.sub.2 O.Al.sub.2 O.sub.3.2SiO.sub.2.6H.sub.2 O, could present varied degrees of solubility in fountain solutions used on printing presses. In addition, if varied cations such as Ca, Mg, etc., are present, they may also form complex double silicates with the aluminum, which may cause further loss in quality of the formed layer.
Silication of aluminum plates by the processes of the prior art requires control of the purity of the solution and of the process variables as closely as feasible, such process variables being the pH of the solution, the concentration of silicate, the temperature of the solution, the duration of the operation, the amount of grain of the plate, the plate surface cleanliness, the degreasing or dismutting processes utilized, etc. If all the process variables are closely controlled in the prior art processes, it is possible to obtain silicated aluminum plates of acceptable quality for use as support members for photo-lithographic plates. The most important of the desirable qualities to be achieved consist in an adequate chemically inert surface layer which does not deteriorate with age and is uniform and well bonded to the aluminum base material and which protects the aluminum surface in such manner that it is prevented from interreacting with the acidic diazo resin and will be only slowly etched by the acidic fountain solutions, and in providing an appropriate anchorage for the light exposed diazo resin which permits the developing lacquer to build up on the image areas and to supply long lasting oleophilicity of the image areas, thus insuring long runs of the plate in the printing press. Such qualities are difficult to obtain in a repetitive manner by way of the processes of the prior art.
The present invention, by contrast, by utilizing an electrolytic process for forming an improved functional surface on aluminum plates and other metallic elements permits to achieve consistent and repetitive quality in the surface and permits to obtain a surface greatly enhancing the quality of photo-lithographic plates as compared to what is achieved by prior art methods.